This invention relates to a method of producing a so-called glazed ceramic substrate by forming a glass coating layer on a major surface of a ceramic substrate by firing the substrate and then cooling the substrate to allow the molten glass layer to turn into a solid coating layer.
Recently there has been an increasing trend to glaze a major surface of a ceramic plate, such as alumina plate, in order to utilize the glazed ceramic plate as a substrate of an electric or electronic device. A ceramic substrate having a glass coating is commonly called a glazed ceramic substrate and features high smoothness of its glazed surface besides the favorable characteristics of the ceramic substrate such as high stability at high temperatures and good workability.
At present glazed ceramic substrates are largely used in manufacturing hybrid integrated circuits. Each glazed ceramic substrate for this use is relatively small in size, and the glass coating of the glazed ceramic substrate is not required to be highly heat-resistant. Therefore, it is not so difficult to select a glass composition suitable as the coating material, that is, to select a glass which can readily be fused onto a ceramic substrate at a relatively low heating temperature and is low in the content of alkali metals, which are unfavorable for the electric characteristics of the coating layer, and comparable in the coefficient of thermal expansion to the ceramic substrate. The comparableness in thermal expansion coefficient between the glass and the ceramic is desired from the viewpoint of obtaining a glazed ceramic substrate high in flatness without suffering from warping of the substrate subjected to cooling from a high temperature during the glazing procedure.
Meanwhile, there is a increasing trend to use glazed ceramic substrates in manufacturing thermal heads of thermal printing devices. Glazed ceramic substrates for this use are generally required to be very high in stability and durability of their glass coating layers at considerably high temperatures, and accordingly it becomes preferable to employ a glass composition which features highness of its transition point as the coating material. Then it becomes difficult to meet the desire of using a glass of which coefficient of thermal expansion is close to that of the ceramic substrate to be coated with the glass, and consequentially warping of the substrates subjected to the glazing procedure becomes a serious problem in industrial production of glazed ceramic substrates having satisfactory high-temperature characteristics. This problem is further augmented by the fact that relatively large-sized substrates are needful for thermal heads.
There is a possibility of suppressing warping of a glazed ceramic substrate by increasing the thickness of the ceramic substrate to be coated with glass, but this method is hardly practicable because it places great restrictions on the design of the thermal heads. For a similar reason, it is also difficult to employ a specific ceramic material which is comparable in thermal expansion coefficient to the glass having desirable high-temperature characteristics.